A study on MoSe2 layer of Mo contact in Cu(In,Ga)Se2 thin film solar cells

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A study onMoSe2 layer of Mo contact in Cu(In,Ga)Se2 thin film solar cells

Yi-Cheng Lin a,1, Ming -Tsung Shen a, Yung-Lin Chen a, Hung-Ru Hsu b, Cheng-Han Wu c

a Department of Mechatronics Engineering, National Changhua University of Education, Changhua, Taiwanb Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwanc Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Taoyuan County, Taiwan

E-mail address: [email protected] (Y.-C. Lin).1 Tel.: +886 4 7232105; fax: +886 4 7211149.

http://dx.doi.org/10.1016/j.tsf.2014.04.0160040-6090/ 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Y.-C. Lin, et al., A studx.doi.org/10.1016/j.tsf.2014.04.016

a b s t r a c t
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Available online xxxx

Keywords:Cu(In,Ga)Se2 (CIGS) thin film solar cellsMoSe2 layerSputtering powerSelenization

This study investigated the influence of sputtering power and selenization on the thickness of theMoSe2, as it re-lates to the performance of Cu(In,Ga)Se2 (CIGS) thin film solar cells with a structure of glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al/Al. When the sputtering power exceeded 200 W (power density of 4.4 W/mm2) or the selenizationtemperature exceeded 773 K, the MoSe2 layer underwent a significant increase in thickness. The use of highersputtering power to deposit the Mo contact resulted in superior Mo crystals and facilitated the formation ofMoSe2 layers with hexagonal close-packed crystal structure during the selenization process. The thickness oftheMoSe2 layer did not increasewith soaking time during selenization. The highest device efficiencywas obtain-ed when the thickness of the MoSe2 layer was 240 nm.

2014 Elsevier B.V. All rights reserved.

1. Introduction

A favorable coefficient of thermal expansion, low resistivity, and rel-ative stability at high temperatures has led to the widespread adoptionof Mo in the fabrication of back electrodes of Cu(In,Ga)Se2 (CIGS) thinfilm solar cells [1]. Producing a CIGS film on the Mo contact can leadto the formation of a MoSe2 layer between the Mo and CIGS duringselenization.MoSe2 layers comprise polycrystalline columnar structurescapable of enhancing the adhesion between the Mo and CIGS [2,3].However, the band gap of MoSe2 (1.41 eV) is wider than that of theCIGS absorber layer, such that a back surface field (BSF) comprising aMoSe2 layer can hinder the recombination of electrons and holes [2,4].The thickness of the MoSe2 layer can be influenced by residual stressin Mo contacts [5], factors in the selenization process [6,7], and charac-teristics of the barrier layer [8]. Residual tensile stress in the Mo layercan increase the thickness of the MoSe2 layer formed during theselenization of the Mo layer [5]. Elevated selenization temperatures[5] and higher concentrations of selenium [7] can also produce MoSe2layers of greater thickness. At present, the degree to which sputteringpower in the formation of Mo contacts influences the MoSe2 layerafter selenization remains unclear. In addition, the resistance of theMoSe2 layer ranges between 101 and 104 cm [9], rendering it a poorconductor; excessively thick MoSe2 layers can adversely affect the per-formance of CIGS solar cells. Optimizing the thickness of theMoSe2 layercould facilitate the formation of a CIGS chalcopyrite phase during theselenization process. Previous researchers determined that the optimal

dy onMoSe2 layer of Mo cont

MoSe2 layer thickness lies between 100 nm and 200 nm [8,10,11];however, those studies did not employ non-toxic Se vapor selenization.This study investigated the degree towhich sputtering power in the for-mation of Mo contacts influences the thickness of the MoSe2 layer afterselenization.We also investigated theperformance of the resulting solarcells in order to determine the optimal thickness for the MoSe2 layer.

2. Experiment

Mo contacts were sputtered onto soda-lime glass (SLG) samples(1 cm 1.5 cm 1 mm) to a thickness of 600 nm, at a work pressureof 0.533 Pa, using a range of sputtering powers: 100 W, 150 W,200 W, and 250 W. Bilayer In/Cu0.7Ga0.3 metal precursor films weredeposited on Mo contacts beginning with elemental In, followed byCu0.7Ga0.3, yielding a CuGaIn film with a total thickness of approx-imately 600 nm [12]. The In metal precursor film was depositedusing evaporation at a temperature of 1273 K and work pressure of2.2 103 Pa. The CuGa metal precursor film was prepared bysputtering at a power of 80 W and work pressure of 0.399 Pa. We theninitiated a two-stage annealing process at 623 K for 20min using varioussoaking times (2040 min) and second-stage annealing temperatures(673823 K) at a ramping rate of 10 K/min. A field-emission scanningelectron microscope (FE-SEM, JEOL JSM-6700F) was used to observethe microstructure of films and measure the thickness of MoSe2 films.X-ray diffraction (XRD, JEOL TF-SEM JSM7000F, CuK, = 1.54052 )was used to investigate the crystalline structure of theMo films. Second-ary ion mass spectroscopy (SIMS, IMS-6f) was used to investigate thedepth profiles of the thin film elements. Extraction voltages were set at10 and 12.5 keV, respectively. Additionally, the current of the O2+ ion

act in Cu(In,Ga)Se2 thin film solar cells, Thin Solid Films (2014), http://
http://dx.doi.org/10.1016/j.tsf.2014.04.016mailto:[email protected]://dx.doi.org/10.1016/j.tsf.2014.04.016http://www.sciencedirect.com/science/journal/00406090http://dx.doi.org/10.1016/j.tsf.2014.04.016http://dx.doi.org/10.1016/j.tsf.2014.04.016

Table 1The composition analysis for the CuInGa metal precursor with various CuGa/In film thickness ratios following selenized annealing.

Thickness of In (nm)/thicknessratio of (CuGa/In)

Compositions (at %) Composition ratio

Cu In Ga Se Cu/III Ga/III

270/0.741 25.5 23.9 1.8 48.8 0.99 0.0820/0.625 23.8 24.6 1.7 49.9 0.90 0.07370/0.541 23.2 25.6 2.2 49.0 0.83 0.07

Fig. 1. Depth profile of each element of annealed CIGS sample.

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was set to 80 and 120 nA, which impacted the surface of the sampleswith 5.5 and 8 kV of energy, respectively.

Following CIGS absorber layer, we performed chemical bath deposi-tion of CdS buffer layers. Highly resistive layers of intrinsic ZnO and

Fig. 2. SEM images of annealed CIGS samples with various sputter

Please cite this article as: Y.-C. Lin, et al., A study onMoSe2 layer of Mo contdx.doi.org/10.1016/j.tsf.2014.04.016

conductive ZnO:Al were then deposited by sputtering. The structureof the resulting device was SLG/Mo/In/CuGa/CdS/i-ZnO/ZnO:Al/Alwithout MgF2 anti-reflection film. Cell efficiency was measured at100 mW/cm2 using an AM 1.5 solar simulator.

3. Results and discussion

3.1. Composition of CIGS absorber layer

Table 1 outlines the results of composition analysis for the CuInGametal precursor with various CuGa/In film thickness ratios followingselenized annealing. As shown in the table, the composition of Ga andSe presented no significant changes under these selenization conditions,whereas the Cu composition gradually reduceswith an increase in In con-tent. When the thickness of the In film reached 320 nm (CuGa/In filmthickness ratio = 0.625), the Cu/III ratio in the CIGS absorber layer was0.90. This result is extremely close to the ideal stoichiometric ratio [13].Furthermore, according to Liang et al. [14], a lower Cu/III ratio will alsoreduce the short-circuit current density (JSC) value. Using these specificparameters, the Ga content was relatively low, which may be due to thesinking of Ga element. We further conducted a depth profile of the ele-ments using SIMS. Fig. 1 clearly illustrates the low Ga content in the sur-face of the film. These findings conform to the results of Marudachalamet al. [15], indicating the presence of CuInSe2 in the surface of the filmand CuGaSe2 at the bottom of the film near the Mo contact.

3.2. Influence of sputtering power of Mo contact prepared

Fig. 2 presents SEM cross-sections of glass/Mo/CIGS samples withMo contacts prepared at various sputtering powers. At sputtering

ing powers: (a) 100 W, (b) 150 W, (c) 200 W, and (d) 250 W.

http://dx.doi.org/10.1016/j.tsf.2014.04.016http://dx.doi.org/10.1016/j.tsf.2014.04.016

Fig. 3. Thickness of theMoSe2 layer and the resistance as function of the sputtering powerof the resulting Mo contact.

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powers under 150W (or power densities under 3.3W/cm2), theMo didnot produce significant quantities of MoSe2, compared to that producedat sputtering powers exceeding 200 W (or power densities exceeding4.4 W/cm2). Fig. 3 illustrates the effect of sputtering power (in the for-mation of the Mo contact) on the thickness of the MoSe2 layer as wellas the resistance of the resulting Mo contact. As shown, the sheet resis-tance decreased with an increase in sputtering power; however, thethickness of the MoSe2 layer increased following selenization. Varyingthe sputtering power altered the microstructures associated with theresulting Mo contact as well as the thickness of the MoSe2 layer andthe resistance of the Mo contact. Results of SEM observation and XRDanalysis of the Mo contacts are presented in the following.

Fig. 4. SEM images of the surface and cross-sections of the Mo contacts deposited a


Fig. 4 presents SEM images of the surface and cross-sections of theMo contacts deposited at various sputtering powers. Images of the sur-face of the Mo contacts reveal that sputtering power of 100W resultedin fish-like grains on the surface of the Mo contact. These featurespresent a porous structure with indistinct grain boundaries and surfacecrystals clearly exhibit the existence of voids. At a sputtering power of250 W (5.5 W/cm2), the fish-like grains on the Mo contact increasedin density, presenting clearer grain boundaries and an absence ofvoids among surface crystals. Cross-sectional SEM images revealedthat increasing sputtering power increased the density of the crystallinestructure. Fig. 5 presents the results of XRD analysis of the Mo contact.The diffraction intensity of Mo (110) and (211) peaks was relativelyweak at low sputtering power, gradually increasing with sputteringpower. These results are in agreement with previous findings [1618].The full width at half maximum of Mo (110) indicates that the qualityof crystals in the Mo contact improved with sputtering power, whichsubsequently reduced the resistance of the Mo contact (Fig. 3). Fig. 3shows that as sputtering power increased, the thickness of the resultingMoSe2 layer also increased. This may be explained by the fact thatincreasing sputteringpower enhances thediffraction intensity and crys-tallinity of Mo (110) and (211) (Fig. 5). This may facilitate the transfor-mation (via selenization) of the body-centered cubic crystal structure ofthe Mo contact into a MoSe2 layer with hexagonal close-packed crystalstructure [11,19].

3.3. Influence of selenization temperature and soaking time

Fig. 6 presents SEM images of the MoSe2 layers formed on theCuInGa precursor following selenization at various temperatures.At selenization temperatures below 723 K, the MoSe2 layers areextremely thin. When the temperature during the second-stage ofselenization exceeded 773 K, the thickness of the MoSe2 layers in-creased substantially. Fig. 7 presents the observed results from Fig. 6.These results are similar to those obtained by Abou-Ras et al. [11].Fig. 8 illustrates how soaking time (second-stage annealing) influencedthe thickness of theMoSe2 layer on theMo contact aswell as the perfor-mances of the resulting solar cells. Increasing soaking time duringselenization did not significantly increase the thickness of the MoSe2

t various sputtering powers: (a) 100 W, (b) 150 W, (c) 200 W, and (d) 250 W.

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Fig. 5. The XRD patterns of Mo contacts deposited at various sputtering powers.


layers; however, extending the soaking to 40 min improved crystallin-ity at the bottom of the CIGS absorber layer (Fig. 8(b)).

3.4. Efficiency analysis

This study produced solar cell devices with a structure ofglass/Mo/CIGS/CdS/i-ZnO/ZnO:Al/Al and measured their efficiency,the results of which are presented in Table 2. A1A4 outline the resultsof CIGS cell devices with Mo contacts produced at various sputteringpowers. The open-circuit voltage (VOC) and JSC produced at sputteringpowers below 150W are relatively low, demonstrating poor efficiency.This is because low sputtering power resulted in thin MoSe2 layers,which reduced the effect of the BSF, thereby promoting the recombina-tion of electrons and holes. The best performance was obtained fromdevices produced at 200W, resulting in a MoSe2 layer 240 nm in thick-ness. Although the resistance of theMo contact produced at 250Wwaslower than that produced at 200W (Fig. 3), the thickness of the MoSe2layer increased to 320 nm. According to Shin et al. [8], increasing thethickness of the MoSe2 layer reduces the VOC, JSC, and fill factor (FF)


values of Cu2ZnSnSe4 solar cell, which can jeopardize solar cell efficien-cy. In Table 2, B1, B2, and A3 display the efficiency of CIGS devicesexposed to selenization at various temperatures. VOC increased withan increase in selenization temperature, possibly due to an improve-ment in the crystal quality of the CIGS absorber layer and reduction incrystalline defects (Fig. 6(e)). Nevertheless, higher temperatures duringselenization also contributed to the formation of a thicker MoSe2 layer(Fig. 7), which prevented JSC and FF from increasing [5]. As a result,the efficiency of the solar cell did not increase when the selenizationtemperature was increased to 823 K. The highest efficiency (5.72%)was obtained when the temperature of second-stage selenization was798 K. A3 and C1 present the results of device efficiency obtainedfrom samples produced under various soaking times during second-stage selenization. When soaking time was extended to 40 min, theMoSe2 layer maintained a thickness of 240 nm; however, VOC, FF, andefficiency increased. SEM images in Fig. 8 reveal that extending thesoaking time enhanced the quality of crystals at the bottom of theCIGS absorber layer, which subsequently increased the VOC. Moreover,the thickness of the MoSe2 layer did not increase with longer soakingtimes. Thus, VOC and FF were not affected; however, the efficiency ofthe solar cell was enhanced. Fig. 9 presents the measurements of JVcurve of solar cell fabricated at a deposition power of the resulting Mocontact of 200 W and annealing process 798 K for 20 min.

At JSC = 40.53 mA/cm2, the performance of the CIGS device inthis study was relatively good, compared with the high-efficiencyCIGS devices in previous studies [8,20,21]. However, our VOC and FFvalues were low, reaching 0.447 V and 39.41%, respectively. Wespeculate that the degraded performance with regard to these valuesis due to the deposition of CuGa on the top and In at the bottom ofthe CuInGa precursor, leading to poor crystallinity at the bottomof the CIGS absorber layer (Fig. 2). According to Hsu et al. [22], poorcrystal quality in the CIGS absorber layer can reduce VOC and FFvalues in solar cells. Furthermore, the SIMS results (Table 1) indicatethat the sinking of Ga resulted in insufficient Ga content on the sur-face of the CIGS absorber layer, leading to a further decrease in VOC.In addition, there are possible secondary phases of InSe2 and In2Se3that appear at the bottom of the absorbing layer [12].

4. Conclusions

This paper reports the influence of sputtering power and selenizationon the thickness of the MoSe2, as it relates to the performance of CIGSthin film solar cells. When the sputtering power exceeded 200 W(power density of 4.4 W/mm2) or the selenization temperatureexceeded 773 K, the MoSe2 layer underwent a significant increasein thickness. The use of higher sputtering power to deposit the Mo con-tact resulted in superior Mo crystals and facilitated the formation ofMoSe2 layers with hexagonal close-packed crystal structure during theselenization process. The thickness of the MoSe2 layer did not increasewith soaking time during selenization. The best device efficiency wasobtained when the thickness of the MoSe2 layer was 240 nm.

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Fig. 6. SEM images of the MoSe2 layers formed on the CuInGa precursor following selenization at various temperatures: (a) 673 K, (b) 723 K, (c) 773 K, (d) 798 K, and (e) 823 K.

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Fig. 8. SEM images of the MoSe2 layers formed on the CuInGa precursor followingselenization at various soaking times of second-stage: (a) 20 min and (b) 40 min.

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Table 2Performance of CIGS thin film solar cells with various sputtering powers of Mo contact as well as selenization conditions.

Sample code Power of Mo contact(W)

Annealing temperature(K)

Soaking time(min)

Thickness of MoSe2 layer(nm)

VOC(V)

JSC(mA/cm2)

FF(%)

(%)

Active area(cm2)

A1 100 798 20 20 0.172 15.52 23.37 0.62 0.407A2 150 798 20 60 0.223 21.46 29.11 1.39 0.428A3 200 798 20 240 0.385 40.53 36.64 5.72 0.399A4 250 798 20 320 0.378 38.05 34.97 5.03 0.417B1 200 773 20 140 0.352 34.94 33.54 4.12 0.418A3 200 798 20 240 0.385 40.53 36.64 5.72 0.399B2 200 823 20 320 0.405 37.95 35.69 5.49 0.383A3 200 798 20 240 0.385 40.53 36.64 5.72 0.399C1 200 798 40 240 0.447 38.21 39.41 6.73 0.394

Fig. 9. The JV curve of solar cell prepared at a deposition power of the resulting Mocontact of 200 W and annealing process 798 K for 20 min (sample A3).



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http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0065http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0065http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0065http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0065http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0125http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0125http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0125http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0125http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0075http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0080http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0080http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0085http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0085http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0085http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0090http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0090http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0090http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0095http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0095http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0095http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0095http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0095http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0100http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0100http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0100http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0100http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0105http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0105http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0105http://refhub.elsevier.com/S0040-6090(14)00420-9/rf0105http://dx.doi.org/10.1016/j.tsf.2014.04.016http://dx.doi.org/10.1016/j.tsf.2014.04.016A study on MoSe2 layer of Mo contact in Cu(In,Ga)Se2 thin film solar cells1. Introduction2. Experiment3. Results and discussion3.1. Composition of CIGS absorber layer3.2. Influence of sputtering power of Mo contact prepared3.3. Influence of selenization temperature and soaking time3.4. Efficiency analysis4. ConclusionsReferences

A study on MoSe2 layer of Mo contact in Cu(In,Ga)Se2 thin film solar cells

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